The advanced lithography process for the fabrication of semiconductor devices favors a light source of shorter wavelength for exposure. A subsequent transition to lithography using extreme ultraviolet (EUV) is regarded promising.
The EUV lithography uses a reflecting optical system. While EUV light has a short wavelength of 13.5 nm, there are available no materials having high transmittance at that wavelength. EUV light is reflected by a Si/Mo multilayer film sputtered on a surface of a substrate of low coefficient of thermal expansion (CTE) material.
Fabrication of defect-free photomasks is one of the outstanding problems that must be overcome before the EUV lithography can be implemented on a commercial basis. While irregularities or defects on the surface of a photomask substrate are permissible in the KrF (wavelength 248.3 nm) and ArF (wavelength 193.4 nm) lithography employing conventional dioptric system, defects of the same order are not negligible in the EUV lithography because of the shortness of exposure wavelength and the reflecting optical system.
The EUV lithography photomask bears defects which are generally divided into three types: 1) surface defects of a low CTE material substrate, 2) defects in a reflecting multilayer film, and 3) defects on a pattern. As the sputtering conditions are ameliorated and the cleaning technology is improved, the number of defects on the EUV lithography photomask is reduced. However, many defects are still found on the polished surface of low CTE material substrate. Toward the commercial implementation of the EUV lithography, it is urgently required to reduce defects on the polished surface of low CTE material substrate.
Titania-doped quartz glass is well known as the low CTE material which is used in the reflecting optical system for the EUV lithography. The addition of a certain amount of titania is effective for reducing the CTE of quartz glass. Because of titania doping, however, technical difficulty arises in manufacturing photomask substrates which meet the requirements of defect-free and high flatness for the EUV lithography application.
When titania-doped quartz glass has a non-uniform titania concentration, it is difficult to manufacture substrates having a high flatness from such glass. Striae may be formed during manufacture of titania-doped quartz glass. For example, if the reactant gas supply is inconsistent (that is, the silicon and titanium-providing reactant gases are not supplied at constant flow rates), and/or if the temperature of the growth face of a titania-doped quartz glass ingot fluctuates due to variations of the flow rates of oxygen and hydrogen gases supplied simultaneously, the titania concentration becomes non-uniform, generating distinct sites, known as striae. When striated substrates are polished, irregularities are formed on the substrate surface because striae are different in reactivity with the polishing fluid and abrasion rate.
On the other hand, as compared with undoped quartz glass substrates, titania-doped quartz glass substrates tend to bear many defects on their surface after polishing and cleaning. The defects on the substrate surface may be divided into two: convex defects resulting from foreign particles left after polishing and cleaning and high-hardness inclusions within quartz glass emerging at the surface; and concave defects resulting from local high-polishing-rate inclusions within quartz glass emerging at the surface.
Titania-doped quartz glass for use as the material from which EUV lithographic members are made tends to bear more concave defects, which interfere with the manufacture of defect-free photomasks.
JP-A 2010-135732 and WO 2010/131662 describe silica glass substrates. Allegedly, it is preferred that concave pits of at least 60 nm be absent on the substrate surface of a surface quality area. However, the method for manufacturing silica glass substrates so as to eliminate concave pits of at least 60 nm is described nowhere. It is more preferred in view of mask quality that concave pits of at least 40 nm are absent on the substrate surface of a surface quality area. However, mask substrates in which concave pits of at least 40 nm are absent are described nowhere, even in Examples. Also means for measuring defects of 40 nm is described nowhere.
WO 2009/145288 describes that silica glass substrates are preferably free of inclusions which are believed to cause concave defects. However, it is not described even in Examples whether or not inclusions are found in silica glass. Also means for detecting such inclusions is described nowhere.
Even in the case of titania-doped quartz glass in which no inclusions have been found by the standard light collimating detection test, concave defects are often found on its surface after it is polished.
For the goal of defect-free polished surface, it has been a common practice to optimize polishing and cleaning conditions so as to reduce convex and concave defects. Also for the purpose of reducing inclusions in titania-doped quartz glass which cause defects on the polished surface, it is known in connection with the indirect manufacture of titania-doped quartz glass to accurately control temperature conditions during heat treatment for homogenizing titania-doped porous silica matrix (TiO2—SiO2 consolidated body), vitrification, and shaping of TiO2—SiO2 glass body.